Vibrio vulnificus is a human pathogen capable of causing death within 2 to 3 days. Although infections apparently can occur in otherwise healthy individuals, case histories suggest that iron plays an important role in the disease caused by this organism. In addition, studies from our laboratories indicate that V. vulnificus may differ from other bacterial pathogens in possessing a unique combination of factors for acquisition of heme iron. Whereas most bacteria are able to utilize free hemoglobin as an iron source, haptoglobin (a normal serum protein) blocks such iron acquisition by immediately and irreversibly binding free hemoglobin. However, we have found that V. vulnificus is able to override this bacteriostatic effect of haptoglobin, and to utilize heme iron. Based upon these observations we propose that the ability to obtain iron in the low iron environments of blood and tissue fluids contributes to the high virulence of V. vulnificus, and that avirulent strains are deficient in one or more of the factors for heme iron acquisition. Our model to describe the interplay of these factors during the infective process is (1) hemoglobin is released from erythrocytes, (2) the haptoglobin-hemoglobin complex is "processed", liberating heme iron; (3) the heme iron is internalized. To test this model we propose to generate from a virulent strain of V. vulnificus mutant strains which are deficient for one or more of the factors indicated in the virulence model. Specifically, mutants to be characterized include those which (1) are unable to produce hemolysin but can extract iron from hemoglobin and the haptoglobin-hemoglobin complex, (2) are unable to produce hemolysin and extract iron from the haptoglobin-hemoglobin complex, but can acquire iron from free hemoglobin, and (3) are hemolysin positive but have lost the ability to extract iron from the haptoglobin-hemoglobin complex. These mutants finally will be characterized as to their relative virulence in mice compared to the parent strain. These experiments would provide strong evidence as to whether the ability to acquire iron from the erythrocyte hemoglobin pool contributes significantly to the virulence of V. vulnificus.